Communication with technical means between a finite number of particular participants is referred to as a communication session. In a communication system, the right to communicate (communication right) is usually controlled by means of a communication right allocation mechanism. As part of this mechanism, the participant in a communication session requests a communication right from a communication session server, and the server approves the right or does not approve it.
By way of example, conference systems allow communication appliances to be used for communication between a plurality of users.
In order to allow ordered communication, usually not all the participants in a conference receive the right to communicate via a particular medium (such as audio, video, text, etc.) at the same time. On the contrary, the communication rights are usually allocated on the basis of certain prescribed rules.
In a large conference room, a conference system is used which provides the participants with a plurality of microphones and loudspeakers for voice communication for example. The microphones need to be switched on by the respective speaker in order to use them. A microphone which has been switched on blocks all other microphones, so that only one speaker can ever be active. By way of exception, it is also possible for a further microphone (for example the microphone of the head of the conference) to be active at the same time. The right to speak is thus only ever given to one participant and possibly to the head of the conference.
A comparable service also exists for mobile radio telephones: Push-to-Talk (PTT). The technical term Push-to-Talk denotes that the speaker usually operates a special key on the mobile radio telephone in order to send messages, as in the case of a “walkie-talkie” (a small radio telephone set). The transmission of messages from other users of this service is blocked during this time. Examples of PTT services are “Direct Connect” from Nextel in the USA and “Push-to-Talk over Cellular” (PoC) from the Open Mobile Alliance (OMA).
In a Push-to-Talk conference system, communication rights are requested and allocated by means of what is known as the “RealTime Control Protocol” (RTCP), for example.
In a conference system as proposed by the IETF (Internet Engineering Task Force) committee, an example of an Internet-based conference system, and in a Push-to-Talk conference system (also called a PTT conference system) today, communication sessions are controlled using what is known as the “Session Initiation Protocol” (SIP), for example.
Conference systems based on the IETF and PTT conference systems usually have a centralized architecture. This means that the participants in such conference systems do not communicate with one another directly but rather communicate with one another via a central communication session server (also called a telecommunication server or telecommunication conference server). The central communication session server is arranged in the non-mobile part of the telecommunication network in the case of a mobile communication system or conference system, as explained in more detail below.
A communication session in a communication system may be active or not active. A communication session is active if the communication system simply provides the necessary means for communication. If the means are not available (for example because the necessary communication channels for transmitting data have not yet been prepared), the communication session is not active.
A communication session in a communication system or conference system is initiated by initiating the setup of the necessary communication means. By way of example, the initiation can be performed by one of the communication session participants by virtue of it instructing a central communication session server to provide the communication means. In this case, the participant notifies the server of which participants are intended to be able to communicate with one another.
The participants in the communication session can be specified by a list of participant addresses or by an address which refers to a list of participant addresses (group address).
What is known as a predefined PTT group is specified in a group server by an address which refers to a list of participant addresses. What is known as a chat group is likewise specified in a group server by an address which refers to a list of participant addresses. In contrast to a predefined PTT group, the members of a chat group are not all invited to initiate a chat communication session at once. On the contrary, the participants in a chat group can join a chat communication session by dialing up separately. What is known as an ad hoc group is specified by a list of participant addresses. The addresses are specified with the invitation to join an ad hoc communication session by the inviter.
A drawback is that a potential participant in communication sessions has no information about ongoing, that is to say, active, communication sessions. He must first of all use his communication session participant appliance to dial up into a communication session or to invite himself to a communication session in order to be able to establish whether other participants are participating or ready to participate in this communication session. If a potential user is interested in a plurality of communication sessions, it is complex and time-consuming for him to find out which communication sessions are currently active. Therefore, he possibly misses a communication session of interest or a part of it.